The objectives of this grant are to study the inductive relationship between developing central and peripheral synapses, and the influence of these synapses on developmental changes in motoneuron properties. To ensure specific and long-term interaction between cells, differentiation of synapses is directed by gene expression as well as by influential signals passing in antidromic and orthodromic directions across the synapse. Detailed information has accumulated describing the function of sensory-motor synapses in the spinal cord of adult mammals, but little is known about their development. Developing central synapses have been intensively studied in invertebrates and non- mammalian vertebrates such as tadpole and chick. This project will focus on studying the electrical, pharmacological and morphological properties of spinal motoneurons of rat embryos, and investigating the roles of developing sensory-motoneuron contacts and peripheral nerve-muscle interactions on these properties. The pattern and time course of development of spinal reflexes and the initial specificity of afferent motoneuron contacts in spinal cord of rat embryos will be examined. These studies will be carried out in vitro using isolated segments of spinal cord. The small size of the embryonic spinal cord allows adequate penetration of oxygen and nutrients into the tissue which remains viable for many hours. Thoracic segments of spinal cord with their adjacent intercostal muscles will be maintained in organ culture for up to 6 days. The advantages of using spinal cord-intercostal muscle explants are: 1) motoneurons are cultured in continuity with the muscles they normally innervate in vivo, and 2) we have previously described in chronological changes that occur during development of intercostal nerve-muscle contacts in vivo and in organ culture, and thus will be able to correlate these changes with the differentiation of motoneurons that innervate them. Maintaining these explants in the controlled environment of organ culture is valuable for determining the signals that influence motoneuron properties and the formation of central neuronal interactions. Understanding the mechanisms involved in specificity of synapses during development may give insight into mechanisms of long- term interactions between cells and reasons for their inability to re-establish these interactions following lesions in the central nervous system.